In order to significantly reduce nitrogen oxide (NO.sub.x) emissions from gas turbine engines, multiple combustion chambers are typically employed within the burner section of an engine. Fuel is introduced to each combustion chamber through one or more respective nozzles, and the sets of nozzles are typically referred to as pilot nozzles and main nozzles. At low fuel flow rates, fuel is fed only to one or more of the pilot nozzles, whereas at higher fuel flow rates, fuel is fed or staged to one or more main nozzles in addition to the pilot nozzles.
Fuel control systems are provided for metering the fuel flowing from fuel pumps to the fuel nozzles. Typically, the fuel pump is a positive-displacement type pump, in which the flow output of the pump is proportional to the pump speed. Because the fuel pump delivers more fuel to the engine than the fuel nozzles require, the fuel control system must bypass any fuel flow that is not delivered to the fuel nozzles back to the fuel pump.
One known system for metering fuel to two sets of nozzles is shown, for example, in U.S. Pat. No. 4,949,538 to Iasillo et al. In this system, a conventional flow metering unit (FMU) meters the total fuel flow required by the engine, and a downstream staging or splitter valve divides the total fuel flow, directing a portion to the pilot nozzles, and the remainder to the main nozzles. When a main nozzle is initially activated, at least a portion of the fuel flow to the corresponding pilot nozzle(s) is temporarily diverted to the main nozzle flow path until the main nozzle flow path is filled with fuel. As a result, although the fuel system is attempting to increase the flow of fuel to the combustion chambers, the overall flow of fuel to the combustion chambers is temporarily reduced. In an aircraft application, this condition is not acceptable.
This type of system also requires complicated logic in order to coordinate the operation of the metering valve of the FMU with the operation of the splitter valve. For example, when fuel flow is increased from a low flow to a higher flow level for staging the main fuel nozzles, the splitter valve must be actuated ahead of the metering valve, in order to prevent a portion of the fuel flow intended for the main nozzles from being delivered to the pilot nozzles during the transient. The pilot nozzles are typically not sized to handle any such relatively large flow, and therefore if fuel flow to the main nozzles were temporarily delivered to the pilot nozzles, a large pressure spike would occur. In similar fashion, when fuel flow is reduced from a relatively high level flowing to both pilot and main nozzles to a relatively low level flowing to only pilot nozzles, the splitter valve must be actuated ahead of the metering valve in order to prevent a flameout from occurring.
It is an object of the present invention to provide a fuel control system for metering fuel flow to at least two fuel nozzles that overcomes the drawbacks and disadvantages of prior art fuel control systems.